Slate floors - subfloor


Old 01-06-08, 03:21 PM
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Slate floors - subfloor

We are replacing our bathroom floor with slate tiles.
The existing floor is linoleum and there is 1/2" pressboard
ontop of the sub-floor (plywood).
My question is, can we put the 1/4" backer board (cement board) ontop of the pressboard or should we remove this board before putting down the backer board?
The floor of the bathroom will be meeting with the existing
oak flooring(3/4" - 1") in the hallway.
I understand the backer board and/or plywood under the tiles will give rigidity to the floor, as to stop any cracking in the tiles and/or groat.
Any information would be appreciated very much.
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Old 01-06-08, 05:40 PM
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You didn't mention how thick the subfloor is, and you didn't mention anything about the joists. That's where we need to start...with the joists system.

As for what you called press board, I have no idea what 'press board' is. You'll need to figure that out. If it's particle board, it needs to be removed. If it's OSB, maybe it can stay?

If your joists system meets requirements for natural stone you need a double layer of subfloor and then a tile backer or a product like Ditra. The entire subfloor system has to meet L720 deflection. This is a way to rate stiffness.

So tell us the type, size, species, grade, spacing and the unsupported span of the joists. Without that nothing else matters for now.

Old 01-07-08, 06:29 PM
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Slate floor - subfloor

Thank you for your help.
The joists are fir 2" x 10", 16" on center, span from outside wall to support beam in center is 14', this is single level home with crawl space. Floor is 3/4" Plywood. The underlayment
would be the particle board as you mentioned.
The room size to be tiled is 7' x 9' . Using 12" x 12" polished
slate tiles.
I am calling this board that tile store has said to use (cement board) which may not be the proper name for it?
I believe it comes in two thicknesses, I know that one is 1/4" and the other thicker, not sure how much thicker?
I hope that I haven't missed anything.
Do people use exterior grade plywood for a additional layer of flooring beneath the cement board?
Is this cement board for rigidity or sealing properties?
Old 01-07-08, 07:22 PM
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Cement board is the correct term, or CBU, hardibacker, etc.

They typically come in 3'x5' sheets in either 1/4" or 1/2" thickness.

The CBU has a few purposes:

Better tile adhesion (plywood sucks the moisture out of the thinset which can cause premature tile failure)

Isolates any slight movement within the subfloor.

Hardibacker, aka cement board needs to be thinsetted down and screwed, not just screwed.

CBU is not meant for strength.

3/4" plywood subfloor like you have is at the minimum recommended level for a natural stone. I would want a little more if it were my room. 1" should be good.

Use a good quality exterior grade plywood for subfloor.
Old 01-07-08, 07:28 PM
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If the unsupported span is 14', you are NOT going to get my blessings to install natural stone. Your joists are over-spanned at least 12", or more. The general minimum code is L360. You are close to L400, you need to be at L720. How did you measure the 'unsupported' span? Many people do this wrong.

Did you measure from underneath? Did you measure from face to face of the supports? If the span was under 13' you might make it?

If the answer is still the same you can do one of three things. Build a supporting wall/structure to shorten the span. You can sister the joists so that they become 3x10's. Or you can install ceramic or porcelain tiles instead.

More on the subfloor layers later.

Old 01-08-08, 07:47 AM
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I agree that you over overspanned for natural stone. You would either have to sister your joists or cut down the span as JazMan said. Additionally, natural stone installations require two layers of plywood so you would have to add another layer of plywood after removing the particle board. Then you would have to add cement board or an isolation membrane over the plywood. This will add extra height to your floor and you will have to decide if that will work for you.

There are some very nice porcelain "slate lookalike" tiles available that would look great and not require you to "beef up" your current framing structure as JazMan described. You are probably ok for ceramic/porcelain but definitely not natural stone. The porcelain will also be much less of a maintenance issue in your bathroom than slate and will look just as good.

The lino and particle board definitely have to go. You could remove them both at the same time.
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